1. Field of the Invention
The present invention generally relates to amplitude-modulation radio-frequency transmissions and, more specifically but not exclusively, to transmissions performed with a modulation index smaller than one.
The present invention applies for example to electromagnetic transponder systems in which a high-frequency carrier is amplitude-modulated by a terminal for transmission to an electromagnetic transponder, for example, carried by an electronic tag or a smart card, in the field of the terminal. The present invention applies, as one possible example, to electromagnetic transponders in an electronic passport application.
2. Discussion of the Related Art
Systems for electromagnetic transponders are based on the cooperation between an oscillating circuit on the read/write terminal side and a resonant circuit on the electromagnetic transponder side (generally, a portable element), to exchange information by using a high-frequency field radiated by the oscillating circuit of the terminal. Often, the high-frequency carrier is also used as a remote-supply carrier providing the transponder supply power.
An example of application of the present invention relates to transponder systems based on standards ISO 14443 and 15693 according to which the remote-supply carrier radiated by the terminal is 13.56 MHz, while a back-modulation sub-carrier may be used by the transponders to transmit information to the terminal with an 847.5-kHz frequency. In the terminal-to-transponder direction, the carrier is modulated in amplitude with a modulation index generally on the order of 10% with a flow rate on the order of 106 kilobits per second. The modulation index defines as being the amplitude difference between the transmission of a state 1 and the transmission of a state 0, divided by the sum of these amplitudes.
FIG. 1 very schematically shows in the form of blocks an example of an electromagnetic transponder system to which the present invention applies. A transponder 1 (TR) is intended to be placed in the electromagnetic field of a terminal 2 (TERM) having an inductive element L2 of an oscillating circuit emitting a high-frequency radiation detected by an antenna L1 of transponder 1.
FIG. 2 very schematically shows, partly in the form of blocks, an example of a conventional architecture of an electromagnetic transponder 1, intended to communicate with a read/write terminal (not shown in FIG. 2). The transponder comprises an oscillating circuit 10, formed of an inductive element L1 forming an antenna, in parallel with a capacitor C1 at the A.C. input terminals of a rectifying bridge 11. The rectified output terminals of bridge 11 are connected by a storage capacitor Cs. As an alternative (not shown), capacitor Cs is replaced with a bias source (for example, a current source or a resistor, or even directly the remotely supplied load).
The signal detected when transponder 1 is in the field of a terminal is used, among others, for extracting a supply voltage Vdd from the transponder circuits, by means of a regulator 12 (REG) and to decode the possible information transmitted by the terminal.
For this purpose, the transponder comprises an amplitude-demodulation circuit comprising, for example, a low-pass filter schematically formed of a resistor R5 and of a capacitor C3, a terminal of resistor R5 being connected to the common node of capacitor Cs and of one of the output terminals of bridge 11 (and thus receives the rectified and filtered voltage Vdc), while its other terminal is connected to the common node between capacitor C3, having its other electrode connected to ground, and a capacitor C4 filtering the D.C. component to provide on its other electrode a signal Ve only containing the edges of voltage Vdc. The cut-off frequency of the high-pass filter (capacitor C4) is selected to let through with no significant dimming the binary data amplitude-modulated on the carrier. Further, capacitor Cs takes part in a low-pass filtering such that rectified and filtered Vdc carries the envelope of the amplitude-modulated signal.
Signal Ve is applied on a first terminal of a resistive element R6, having its other terminal connected to ground by a capacitor C5. Signal Ve is further applied to first respective inputs (for example, non-inverting and inverting) of two comparators 13 and 14 with two thresholds +ref and −ref. Thresholds +ref and −ref are obtained by a dividing bridge formed, for example, of four resistive elements R1, R2, R3, and R4 in series between two terminals of application of voltage Vdd, the junction point of resistors R1 and R2 providing voltage +ref, the junction point of resistors R2 and R3 providing an average value Vm to the second terminal of resistor R6, and the junction point of resistors R3 and R4 providing level −ref. Average voltage Vm depends on the operating ranges of comparators 13 and 14 and/or on the downstream circuits. In the example, the average voltage corresponds to half Vdd/2 of the supply voltage.
The dividing bridge and resistor R6 enable setting a D.C. component to value Vm for signal Ve carrying the modulation edges only.
The outputs of comparators 13 and 14 are connected to the respective S input for setting to 1 and reset input R of an RS-type flip-flop 15 having its output D providing the detected (demodulated) state to a digital interpretation circuit 16 (for example, an arithmetical and logic unit UART).
A demodulator such as shown in FIG. 2 is described, for example, in U.S. Pat. No. 6,031,419.
To simplify the representation of FIG. 2, account has only been taken of the receive portion of the transponder. In particular, the back-modulation elements of the load formed by the transponder in the electromagnetic field of a terminal for a transmission in the terminal-to-transponder direction have not been shown. Further, a signal (not shown) is directly sampled from oscillating circuit 10 of the transponder to detect the presence of a radio-frequency signal and extract a clock from the carrier.
FIGS. 3A, 3B, 3C, and 3D illustrate the operation of the demodulator shown in FIG. 2. FIG. 3A shows an example of the shape of a signal Vlc across oscillating circuit 10 received from a terminal. FIG. 3B illustrates the shape of signal Vdc at the output of rectifying bridge 11 (upstream of filter 13). FIG. 3C illustrates the shape of signal Ve applied on the comparison inputs of comparators 13 and 14 and the comparison thresholds −ref and +ref set by resistors R1, R2, R3, and R4. FIG. 3D illustrates the result provided by the D output of flip-flop 15.
As illustrated in the left-hand portion of these drawings, rectified signal Vdc is, in principle, above or under its average value Vm according to state 1 or 0 of the transmitted bit. Capacitor C4 filters the D.C. component, so that signal Ve applied to the comparison inputs of comparators 13 and 14 only comprises, around value Vm, the edges on state switchings. The comparators detect when the signal comes out of the window defined by thresholds −ref and +ref, and flip-flop 15 provides a state 0 or 1 according to the direction of the detected threshold.
A problem is posed if the amplitude-modulation transmission flow rate is desired to be increased in the terminal-to-transponder direction. This problem is illustrated in the right-hand portion of the timing diagrams of FIGS. 3A to 3D. The state switchings of the envelope of the signal received around value Vm (FIG. 3B) are closer than in the left-hand portion of the timing diagrams.
As illustrated in FIG. 3C, signal Ve does not have time to come back to median value Vm before occurrence of the next edge of the modulated signal. Since only the edges are transmitted by capacitor C4 and the amplitude of these edges depends on the modulation index, the system becomes incapable of detecting subsequent switchings as long as the state has not been maintained for a sufficient time to enable signal Ve to recover quiescent value Vm.
This disadvantage results in that transponders are, in practice, limited to rates of from 100 to 400 kbits/s. Now, more and more applications require more significant flow rates (for example, electronic passports for the transmission of biometric data or multiple-application transponders).
Reducing the detection window set by thresholds −ref and +ref is no solution to this problem since the demodulator would become too sensitive to noise.